This invention is concerned with surface protection of halide solids. In particular, the present invention is concerned with the surface protection of water soluble halide solids for use as optical components in infrared systems.
One of the more critical problems encountered in the development of high power infrared lasers is the development of laser windows which are highly transparent to laser radiation at 10.6 microns and at 3 to 5 microns. At the present time, considerable research effort has been devoted to the development of laser windows from the so-called covalent compounds consisting typically of II-VI compounds such as cadmium telluride, zinc telluride, and zinc selenide. The need for improved laser window materials, however, is well known. See, for example, F. Horrigan, et al, "Windows for High Power Lasers" Microwaves, page 68 (January, 1969); M. Sparks, "Optical Distortion by Heated Windows in High Power Laser Systems", J. Appl. Phys., 42, 5029 (1971).
The need for improved laser windows is based on the extremely high laser power throughput required and the fact that laser windows constitute structural members. In order to maintain high throughput and minimize adverse effects, the amount of energy transferred to the window must be kept low. Laser beam energy can be transferred to the window in two ways: heating of the window caused by either bulk or surface absorption of the beam, or direct conversion of the beam energy to mechanical energy by brillouin scattering or electrostriction. This energy transfer produces several undesirable effects such as lensing and birefringence, which result in degradation of beam quality and polarization. In extreme cases, severe thermal stresses can be produced in the windows. These stresses, which are further aggravated by the fact that the windows are mounted in a cooling clamp, may lead to fracture of the windows.
The low absorption coefficients of the halides make then outstanding candidates for optical components in infrared systems. The alkali halides exhibit low absorption from the near ultraviolet to beyond 10.6 microns, and the alkaline earth halides exhibit low absorption in the 2 to 6 micron region. Furthermore, because the temperature coefficient of the index of refraction and the coefficient of thermal expansion have opposite signs, the two effects tend to compensate optical path changes due to temperature, making these materials useful in applications in which heating by a laser beam is anticipated.
Two fundamental problems with the halides, however, have limited their use as high power laser windows. First halide crystals have low yield strengths and are highly susceptible to fracture. Second, many halides, in particular the alkali halides, are water soluble and cannot, therefore, be used in humid environments.
The first problem has recently been overcome. Techniques for strengthening halides by hot working without altering their optical properties have been developed. These techniques are described in U.S. Patent Applications Ser. No. 634,394 filed Nov. 24, 1975 which is a continuation of Ser. No. 445,371 filed Feb. 25, 1974 and now abandoned; Ser. No. 619,264 filed Oct. 10, 1975 which is a continuation of Ser. No. 445,394 filed Feb. 25, 1974 and now abandoned; and Ser. No. 617,350 filed Sept. 29, 1975 which is a continuation of Ser. No. 445,393 filed Feb. 25, 1974 and now abandoned. These patent applications are assigned to the same assignee as this application.
Despite extensive research efforts, the second problem, surface protection, has not previously been satisfactorily overcome. Conventional coating methods for sealing the surface of the halide solid from environmental humidity have generally failed for one of two reasons. First, the coatings delaminate during thermal cycling because of differences of coefficient of thermal expansion between the coating material and the substrate. This is a serious problem because the large coefficient of thermal expansion of halides tends to result in coatings that are in tension. It has not been uncommon for the protective coating to peel off of a halide window during use. Second, the coating material is sufficiently opaque in the infrared to negate the extremely low optical loss which makes the halides attractive.
In the previously mentioned co-pending application now U.S. Pat. No. 3,959,548 by Enrique Bernal G. entitled "Graded Composition Coating for Surface Protection of Halide Optical Elements", the shortcomings of the prior protective coatings have been overcome. The protective layer taught in that application comprises an alloy of a first water soluble, halide material and a second material which is essentially insoluble in water. The composition of the protective layer varies from essentially the first halide material at the interface of the protective layer and the body to be protected, to the second material at the opposite surface of the protective layer.
Initial experiments which attempted to prepare protective layers of the type described by Enrique Bernal G. utilized thallium iodide (TlI) -- potassium chloride (KCl) protective layers on KCl substrates. Thallium iodide was chosen as the second material because it has a very low absorption coefficient at the wavelengths of interest (10.6 microns) and is essentially water insoluble. Graded composition TlI-KCl protective layers were prepared on KCl substrates by vapor deposition. KCl and TlI were co-deposited in a vacuum from two independently heated crucibles. KCl alone was first deposited onto a cleaved single-crystal KCl substrate at room temperature. After a period of time, the KCl deposition was reduced to zero. During this transition interval the TlI deposition was smoothly increased to its predetermined steady state value and maintained for the remainder of the deposition run.
The films prepared by this method exhibited absorption coefficients at 10.6 microns of about 25 cm.sup.-.sup.1. This was much higher than for pure TlI films discretely deposited on KCl, which had exhibited absorption coefficients of less then 1 cm.sup.-.sup.1 at 10.6 microns.